Common medium multichannel exchange system



1936- L. ESPENSCHIED El AL 2,064,895

COMMON MEDIUMMULTICHANNEL EXCHANGE SYSTEM Filed May 26, 1934 2Sheets-Sheet 2 12': Other Jelectz'rgg (its. Z?) Other Sclectzi'zy Cats.

(V6 ice Hague/24y, Radio, m're Cam'er or Other ('luvmels) Central Officex INVENTORS LEs aenJcMd,iu Y BY /lZ/Yfonk ATTORNEY Patented Dec. 22,1936 UNITED STATES PATENT OFFIQE COMlVION MEDIUM MULTICHANNEL EXCHANGESYSTEM tion of New York Application May 26, 1934, Serial No. 727,808

14 Claims.

This invention has for its object the provision of an exchange system inwhich communication between subscribers is carried out over channels ofdifferent frequency derived from a common transmission medium. Moreparticularly, the invention is designed to utilize as a substitute forthe many subscriber pairs which are employed in the ordinary telephoneexchange system the many channels which are obtainable in the highfrequency art as it is now unfolding and to provide means whereby thesechannels may be employed by subscribers for interconnections.

In the telephone exchange systems which have heretofore been provided ithas been the practice to group subscribers in central omce areas. Eachsubscriber is connected to his own central office by means of anindividual wire circuit or by a party line arranged for nonsimultaneoususe by a few subscribers. When a subscriber desires to call anothersubscriber he utilizes his own line to the central office, and uponpassing the desired number either to an operator or to an automaticswitching mechanism is connected to another line which extends eitherdirectly or via another ofiice or offices to the called party. For asystem of this kind many thousands of subscriber circuits are necessaryin each central oflice area. These are commonly provided in the form ofa network of multiple pair cables extending over the area.

In accordance with the present invention, it is proposed to substitutefor such individual wire circuits extending to each subscriber atransmission medium which is common to all subscribers and which willaccommodate a range of frequencies sufficiently Wide for a large numberof telephone channels. Interconnection between subscribers is thencarried out over these high frequency channels.

The essence of the invention resides in making available to a largenumber of subscribers a transmission medium capable of accommodating awide range of frequencies so that the subscribers may utilize, for thepurposes of intercommunication, a plurality of signaling channelsderived from the wide band medium. A transmission medium capable ofhandling the large number of channels required for the exchange systemof the invention may be obtained by utilizing either radio transmissionin space or high frequency transmission over a conducting or guidingmedium.

This type of system is not known to have been heretofore invented,apparently because the necessary wide frequency spectrum which isrequired has not been available'to the art either by radio or wiremethods. A conception of the band width which is required may be had byassuming that there is taken as a unit a local exchange system of 1,000subscribers and that each subscriber is allotted an individual channelof 10,000 cycles, making a total band width of some 10,000,000 cycles.Any such band width as this has not been available in the art until veryrecently.

In the case of radio, until the recent opening 0 up of the ultra-highfrequency (ultra-short Wave) range, the art has not permitted theinvention to be realized, for three reasons: First, there was notsufi'icient space in the frequency range, which until recently did notgo beyond about 30 mc., to accommodate this type of service along withall the other demands upon the radio spectrum; second, thecharacteristics of the waves vary widely through this spectrum (up tomegacycles), so that it is difficult to obtain a wide band havingsufficiently uniform characteristics for the purpose; third, the wavesin this range do not cut off very sharply with distance, and as a resulttheir interfering eiTects extend to great distances, in many casesthousands of miles, making it impossible to operate a local service inone area without giving rise to interference in distant areas. With theopening of the ultra-high frequency range, frequencies above 30 mo.(wave lengths below about 10 meters), an entirely new prospect ispresented, making it for the first time physically possible to set up byradio the system comprising the invention. Of course, this is not sayingthat the system will either prove to be desirable or economical foractual operation. This will not be known until a great deal ofdevelopment work is done and the general art is much further advanced.

It will be appreciated also that the invention has not been feasibleheretofore as a wire proposition because it has not been possible untilvery recently to transmit over wire circuits for considerable distancesfrequency bands greater than some tens of thousands or, at most,hundreds of thousands of cycles. The transmission of bands 5 of somemillions of cycles as required by the invention has become possible bywire methods only recently through the development of radically newtypes of wire systems, as exemplified by the coaxial conductor type ofline, as described in U. S. Patent No. 1,835,031, to L. Espenschied andH. A. Afiel. These new wire systems are characterized by the provisionof means for quite completely shielding the transmission path fromoutside disturbances. This is done in the 00- 55 axial circuit by theouter conductor which serves both as a shield and one of the lineconductors. By virtue of the skin effect at these very high frequencies,the desired transmission is confined to the inner surface of the tubeand interference originating from without is confined to the outersurface. Thus, the shielded type of circuit permits of blocking off awide frequency band for the purposes of the invention and subjecting thetransmitting medium to full control in respect to the exclusion ofinterference, the prevention of overhearing on the part of outsiders andthe general knitting together of the system as a selfcontained unit. Inother words, in this wire embodiment of the invention there are retainedthe advantages of wire transmission, plus the advantage of radio inrespect to the availability of a wide frequency spectrum.

In general, these recent developments, whereby the frequency range ofradio and of wire transmission is being greatly extended, are the basisof the present invention. The extension of the frequency range is sogreat as to amount to the imparting to the art of a new dimension ofdevelopment, the frequency dimension, whereby it becomes possible toderive large numbers of telephone channels and to handle them as agroup. Thus, there is obtained on a frequency basis large numbers ofidentical channels comparable to the large number of physical circuitswhich are obtained in the local telephone plant in the familiartelephone cables.

Considering, first, the use of radio as the medium for transmitting thewide band of frequencies contemplated by the invention, the invention isdesigned to employ whatever part of the frequency range may beavailable. Heretofore, however, the radio frequency spectrum hasscarcely afiorded sufficient frequency space to yield the channelcapacity required for a local telephone exchange system. With theopening up of the field of ultra-short waves this limitation as tochannel capacity is removed. Thus, for example, between one and twometers there is a total frequency range of megacycles while the rangebetween 10 and 20 centimeters is 1,500 megacycles. The frequency rangesobtainable through the use of still shorter waves are much greater.

With these ultra-high frequencies, however, nature imposes anotherlimitation, i. e., the tendency of the waves to take on quasi-opticalcharacteristics, so that they project only in straight lines and do notbend around corners to any considerable extent. Thus, it becomesnecessary that subscribers utilizing such waves have a fairly clear lineof sight between their antennas. Some diffraction around obstacles may,of course, be obtained, but, in general, reasonably clear lineof-sightprojection without intervening obstacles appears to be called for. This,of course, has the advantage of making it possible to utilize the sameultra-high frequencies in different areas.

The line-of-sight characteristics of the ultrashort waves must berecognized in the design of a telephone exchange system utilizing thistechnique. In one form of the invention it is proposed to provide in thecenter of a community a central station with antennas mountedsufficiently high above the surrounding buildings to make possible thenecessary directness of transmission between these antennas andpractically any point in the territory to be served. One subscribersstation is then placed in connection with another by transmitting firstto the Central station and thence through the central station to thedesired other subscriber located in another part of the community. Thecentral station acts as a means for catching the waves from onesubscriber at a relatively high point and transmitting, them down againto a second subscriber, thus lift-- ing the transmission path above anyobstacles lying between the subscribers antennas.

In order that such a central station may be economical, it is proposedto make it capable of handling simultaneously a wide band of frequenciessuch as would accommodate hundreds or perhaps thousands of channels. Theparticular arrangement of the central station will be hereinafterdescribed.

As an alternative to the use of a radio transmission medium, theinvention contemplates the employment of a guiding or conductingtransmission path suitable for the required band of frequencies. Such atransmission path would be provided in the form of a network, extendingto all the subscribers within a given area. Conceivably, such a networkmight be composed of ordinary wire circuits branched and interconnectedso as to make the common circuit available to each subscriber. Thus, thewire network might be somewhat similar to the network employed fordistributing light and power currents, except that a simple two-wirenetwork without multi-phase connections, voltage transformationarrangements, etc., would sunice. It would, however, be possible tocarry out the invention by employing the light and power network itselfand this is contemplated within the scope of the invention.

The preferred form of guiding transmission medium, however, is one whichis designed for the transmission of a wide band of frequencies withcomparatively low attenuation and preferably also with shielding tominimize the effect of external disturbances. Such a transmission mediummay be found in a circuit consisting of two conductors disposedcoaxially with respect to one another. In a properly designed coaxialcircuit, low attenuation at high frequencies may be attained by theemployment of conductors of suitably low high-frequency resistance andthe use of a substantially gaseous dielectric between the conductors.With this type of circuit the outer conductor serves also as a shieldwhose protective effect becomes more nearly perfect as the frequency isincreased, so that at high frequencies practically complete immunityfrom external disturbances may be obtained.

The invention contemplates also as an alerna- -tive to the coaxialcircuit a high frequency transmission medium comprising a circuitconsisting of two conductors surrounded by a shield. Here, again, thehigh frequency attenuation may be minimized by proper design withsubstantially gaseous insulation and the thickness of shield may bedetermined so as to minimize interference from external disturbances.

Yet another form of transmission medium suitable for carrying out theinvention is a dielectric wave guide comprising a cylindriformdielectric material which may or may not be surrounded by a conductor.An advantageous form of such a transmission path is a hollow cylindricalconductor containing air or some other gas as the dielectric. This typeof circuit is particularly adapted to transmit waves of very highfrequencies, for example, of the order of a few centimeters or less inwave length.

Using any of the above types of transmission media, the interconnectionof subscribers at will from the available channels involves a number ofnew and difficult problems which are solved by the various features ofthe invention as hereinafter disclosed.

In general, the invention contemplates the assignment to each subscriberof two particular frequencies which might preferably be related to histelephone number. The assignment of frequencies should be such as tofacilitate the interconnection of subscribers and one scheme offrequency allocation is suggested in the invention for this purpose.

The foregoing outline having indicated some of the principles of theinvention as well as a few of its major objects, the complete inventionwith its various details, features and purposes will now be understoodfrom the following description when read in connection with theaccompanying drawings, in which Fig. 1 shows a schematic representation'of the principal idea of the invention wherein a plurality of stationsare connected to .a common transmission medium, Fig. 2 shows inschematic form a system for connecting subscribers through a centralofiice using space radio transmission; Fig. 3 shows one possibleallocation of frequencies for a multi-channel exchange system; Fig. 4shows a subscriber station arrangement which may be used in the systemof Fig. 2; Fig. 5 shows a modulating device which may be used in variousapplications of the invention; Fig. 6 shows a central office arrangementwhich may be used in association with the subscriber station arrangementof Fig. 4; Fig. '7 shows an antenna arrangement for use in a system ofthe type described; Fig. 8 is a schematic diagram indicating howconnections between stations in different areas may be completed throughcentral stations over trunk circuits; and Fig. 9

is a schematic diagram of a guiding or conducting network'which isavailable in common to a central station and a number of individualstations.

Referring to Fig. 1, there is illustrated one of the principal aspectsof the invention. In this figure is shown a common medium CM. to whichare connected a plurality of stations S1, S2, S3, etc., each of whichincludes transmitting, receiving and associated apparatus. Thisapparatus, which will be hereinafter described, is arranged to permitintercommunication between different pairs of stations over the commonmedium CM employing different frequency bands to permit simultaneouscommunication between different pairs of stations. Connection betweenany pair of stations, for example, S1 and S3, takes place through thecommon central station CS.

Fig. 2 shows in schematic form a telephone exchange system in which anumber of subscribers may be interconnected through a central office,the connection between each subscriber and the central o ifice beingaccomplished by radio transmission through space. Each subscriber isprovided with transmitting apparatus including 'an oscillator-modulatorand receiving apparatus including a demodulator, together with thenecessary selecting circuits. At the central ofiice a plurality ofcircuits such as the one shown are provided for interconnecting betweensubscribers. Each connecting circuit includes two separate sets ofapparatus, each of which comprises a radio transmitter and receiver, anda connection to which the operator has access, between the two sets ofapparatus.

Transmission from subscriber A to subscriber B is effected by means ofthe transmitter 'STi,

the oscillator-modulator SM1, the tuned circuit or filter SFi, theantenna SA1, the antenna 0A1, the tuned circuit or filter OFl, thedemodulator ODl, the hybrid coil 0H1, the connecting circuit CC (acrosswhich is bridged the operators set OS), the hybrid coil 0H2, theoscillator-modulator 0M2, the tuned circuit or filter OFi, the antenna0A2, the antenna 5A2, the tuned circuit or filter SE1, the demodulatorSDz and the receiver SR2. Transmission in the opposite direction isaccomplished in an analogous manner, employing the transmitter S'I'z,the oscillatormodulator SMz, the selecting circuit SFa, the antenna SAz,the antenna 0A2, the selecting circuit OFs, the demodulator ODz, thehybrid coil 0H2, the connecting circuit CC, the hybrid coil 0H1, theoscillator-modulator 0M1, the selecting circuit OF2, the antenna 0A1,the antenna SAi, the selecting circuit SFz, the demodulator SD1 and thereceiver SR1.

It is proposed that different frequencies be employed for the oppositelydirected transmissions in Fig, 2 and that the frequencies employedbetween the calling subscriber and the central ofiice be different fromthose used between the central oflice and the called subscriber.Transmission might be on either a double or a single sideband basis withthe carrier frequency transmitted.

In order that simultaneous conversations may take place between a numberof pairs of subscribers, it is necessary that the frequencies employedin the system of Fig. 2 be properly selected, One way of arranging thefrequencies would be to assign to each subscriber a separate pair offrequencies, one for transmitting and one for receivmg, and to provideapparatus at the central oflice for receiving and transmitting thevarious required frequencies. Thus, subscriber A m ght be assigned atransmitting frequency 11 and a receiving frequency f2, and subscriber Btransmitting and receiving frequencies is and f4, respectively. Thiswould mean that in Fig. 2 the modulators SM1 and SMz would operate withcarrier frequencies f1 and f3. respectively, and the modulators 0M1 and0M2 with carrier frequencies )2 and f4, respectively.

With such a scheme it might prove desirable. in order to facilitate theallocation of frequencies and the interconnecting of subscribers, tohave a constant difference between the transmitting and receivingfrequencies of each subscriber or, in other words, to make Thefrequencies f1, f3, etc., might then be grouped in one part of thefrequency spectrum and the frequencies f2, f4, etc., in another part. Adiagram of a frequency allocation of this sort, assuming a total of 99subscribers, is given in Fig. 3. It will be noted that the order of thefrequency assignments corresponds to that of the subscriber numbers.This scheme, while not essential to all embodiments of the invention, isa very desirable feature.

Detailed arrangements of apparatus which might be used at thesubscribers station and at the central office in a scheme of this sortare illustrated in Figs. 4 and 6. While some of the apparatus shown inthese figures is more partlcularlyadapted to radio frequencies lyingbelow the ultra-short wave range, it will be understood that suchapparatus is shown merely to illustrate the principles of the invention,and is not intended in any way to limit its scope.

The operation of the subscribers apparatus illustrated in Fig. 4 is asfollows: When the subscriber desires to make a call, thereceiver SR1 isremoved from the'switchhook SH. Through the contacts of the switchhookthis closesa circuit which furnishes power supply to operate the oscillatorand modulator C and MO. The operation of the switchhook alsoprovides D. C. supply for the transmitter STi, this supply being derived1 through a rectifier RF which is connected to the A. C. supply. Thespeech currents produced in the transmitter, after passing through thetransformer TR1, are modulated upon the assigned frequency, here assumedto be f1. The oscillator and-modulator may be separate units of anydesired types, or a self-oscillating modulator may be used. The carrierand sidebands in the modulator output, after passing through theselecting circuit SF1, are radiated by the antenna SAi to the centralofiice. When communication with the central ofiice is established in amanner hereinafter to be described, the'number of the called subscriberis passed orally to the operator and after the proper arrangements areset up at the central ofiice, communication is established With thecalled subscriber who is provided with apparatus identical with that ofFig. 4 except for the frequency assignments used.

In the case of an incoming call, the carrier.

frequency f2 and its associated sidebands are received by the antennaSAi, selected by the selecting circuit SF2 and demodulated by the demodulator SD1. the vacuum tube type, in which case it should be keptalways in operating condition so that a call may be received.Preferably, however, the demodulator might comprise some passive elementor elements, as, for example, copper-oxide units as shown in Fig. 5.Associated with the demodulator is a relay RL1 which is operated by therectified current resulting from the incoming carrier frequency andwhich in turn operates the call bell SB. When the subscriber answers thecall, the switchhook SH is operated, which, in addition to completingthe contacts previously-mentioned, operates the slow-release relay RLz,which breaks the circuit of the bell SB. -The received voice frequencycurrents, after passing through the transformer TRz, are applied to thereceiver SR1 and two-way communication is thus established. If desired,a low-pass filter LP (shown dotted in Fig. 4) may be included in thereceiver circuit to suppress unwanted high frequency components in thedemodulator output. It will be understood that a similar filter may beprovided in the arrangements hereinafter described. When the subscriberhangs up, the slow release feature of RL2 prevents his bell from ringingbefore the other party also hangs up.

Fig. 6 illustrates central office apparatus which may be used inconjunction with the subscribers apparatus shown in Fig. 4. Theapparatus is so arranged that a call coming in at any frequency isautomatically picked up and brought to the operator, who completes theconnection to the called subscriber. Referring to Fig. 6, the antenna0A0 is designed to receive incoming carrier frequencies within a rangecorresponding to all of the subscribers assigned to the central officeor to a group of subscribers. The received frequencies are delivered tothe tuned circuit OF0 whose frequency of response is being continuouslyvaried by the driving motor DM0. The same motor also rotates the switchSW0 which is provided with a number of circuit 01%. contactcorresponding to any given subscriber is This demodulator may be ofcontacts, one for each subscriber whose incoming frequency may beselected by'the selecting The switch is arranged so that the made at thesame moment that the frequency of that subscriber is picked out by theselecting circuit.

The carrier frequency of an incoming call is selected momentarily by OF0and is rectified in the rectifier OD0. The resultant current operatesthe relay 0R0, which closes a circuit through the winding of the slowrelease relay 0R1, the back contact of the relay CR2 and the switch SW0.The operation of the relay 0R1 lights the subscribers line lamp 0L1thereby calling the attention of the operator to the incoming call.Similarly, as the selection and switching are being varied, the linelamps corresponding to other incoming calls will be lighted.

When the operator observes the lamp 0L1 she inserts a plug, for example,0P1, which is associated with one end of a cord circuit, in thesubscriber's jack 0J1. This action completes a circuit through the relay0R2, whose operation deenergizes the relay CR1 and thus extinguishes thelamp 0L1. After insertion of the plug in the jack OJ 1, the operatoradjusts manually by means of the control 0T1, the response frequency ofthe variable selecting circuit OF1 to the transmitting frequency of thecalling subscriber, which at the same time brings the oscillator 001 andthe selecting circuit OFz to the receiving frequency assigned to thecalling subscriber. The incoming carrier frequency of the callingsubscriber is rectified in the demodulator CD1 and operates the relay0R3, thereby lighting the supervisory lamp 0L2 during the time when thecalling subscribers receiver is off the hook.

Having ascertained from the calling subscriber the number which isdesired, the operator completes the connection by inserting the othercord circuit plug 0P2 in the jack 0J2 which corresponds to the number ofthe called party and at the same time adjusts by means of the manualcontrol GT2 the frequencies of the oscillator 002 and the selectingcircuits OF3 and OF4 to the frequency assignments of the calledsubscriber. The connection established by the jack 0J2 energizes therelay 0R6, thereby preventing the line lamp of the called subscriberfrom lighting when he answers the call. The supervisory lamp 0L3 islighted when the called subscriber answers, and remains lighted until hehangs up. 0 The conversation then proceeds with the apparatusfunctioning in a manner similar to that described in connection withFig. 2. When the conversation is completed the lamps 0L2 and 0L3 areextinguished as the subscribers'hang up and the operator takes down theconnection. By means of keys such as 0K2 the operator may connect herset to different cord circuits and using' the key 0K1 she may split theconnection and talk to either subscriber without the other.

As noted, the selecting circuit OF0 and the switch SW0 might serve forall the subscribers assigned to the office, or these subscribers mightbe divided into groups, with one selecting circuit and switch takingcare of each group, and all such circuits and switches driven by acommon motor.

It is apparent that the arrangement of Fig. 6 permits the employment ofa relatively simple much as the frequencies used by the subscriber arealways the same. The cord circuit arrangement at the central ofiice issomewhat complicated, but the number of cord circuits required will beonly a small fraction of thetotal number of subscribers.

It will be appreciated in connection with Fig. 4 and Fig. 6 that otherknown methods of modulation and demodulation may be employed.Particularly, direct modulation and demodulation whereby the modulationprocess takes place in the electro-acoustical device itself arecontemplated within the scope of the invention. An example of thismethod of modulation using a carbon transmitter may be found in chapterVIII, Fig. 1 of Principlesof Radio Communication by J. H. Morecroft. Acondenser transmitter may be similarly employed as a modulator.

In the various arrangements shown in Fig. 2 to Fig. 6, radiotransmission between the subscribers and the central office may beaccomplished directly using any suitable type of antenna. However, inorder to permit the use of ultra-short waves, for which substantiallylineof-sight projection may be desired, it may be necessary to carry thetransmission path above any obstacles lying between the antennas ofindividual subscribers. For this purpose an arrangement of the typeshown in Fig. 7 may be employed.

Referring to Fig. 7, it will be seen that in this case a single centralofiice antenna A serves for receiving all frequencies from subscribersand transmitting all frequencies to subscribers. This antenna may bemounted on a tall building or tower so that substantialy a line-of-sighttransmission path is obtained between the central oifice and eachsubscribers antenna within the central ofiice area. The subscribersantenna should preferably be designed for radiating a beam toward thecentral ofiice antenna and for receiving radiation from that antenna.Thus the subscribers antenna might be a balanced doublet SA as shown inFig. 7, placed near the focus of a parabolic reflecting surface, PR,directed toward the central office antenna 0A. The central officeantenna, however, is designed for radiating to and receiving from allsubscribers. For this purpose there might be employed an antenna capableof radiating in all directions and receiving radiations from alldirections throughout the range of frequencies utilized. Such anantenna, for example, might be a balanced doublet 0A as shown in Fig. 7.Another possibility would be to employ a central office antenna sodesigned that it is capable of radiating into and receiving from aconical space which comprehends all subscribers locations. This might beaccomplished by adding the reflector RE above the doublet antenna OA.

It is also contemplated that instead of employing a single centralofiice antenna a multiple antenna arrangement might be used at thecentral office building, each antenna being designed for handling a partof the total range of frequencies. It would also be possible to. employtwo antennas for each subscriber instead of one.

Although the arrangements described in connection with Figs. 2, 4, and 6are designed to provide a radio connection between the subscriberswithin a given area, it is proposed to interconnect such stations withsubscriber stations in other areas by providing in each area a centraloffice, so that a radio connection may be lish d be w en a sub c ibe nd.h s

central ofiice, using a frequency or. frequencies assigned for thatpurpose, and the. connection completed over suitable inter-officetrunks. The method is illustrated schematically in Fig. 8. The centralofiice apparatus to be used in carrying out this method might bepatterned after that shown in Fig. 6. The types of trunks which may beused to interconnect the central offices are well known in the art.

In the arrangements thus far described, with the exception of Fig. 1, ithas been assumed that free space is employed as the common transmittingmedium. It is contemplated, however, that transmission over a highfrequency conducting or guiding path may be employed instead of radiotransmission. Such a path would take the form of a network extending toall subscribers within a given area and, to the central office for thatarea. A network of this kind is shown schematically in Fig. 9, whereeach line represents a complete circuit, C designates the central officeand S a subscribers station.

Probably the simplest form of such a network would be one comprised ofordinary two-con ductor circuit suitably interconnected and branched sothat the common circuit will be available at all desired points. Thecircuits comprising the network might, for example, con-'- sist of pairsof open-wire or cable conductors.

It is contemplated also in accordance with the invention that the wirenetwork employed for distributing light and power currents might be usedas the common medium from which to derive high frequency channels forthe telephone exchange system. The high frequency channels would besuperposed upon the power network by carrier methods which are wellknown in the art.

Another form of transmission medium, and one which is peculiarlyadvantageous in that it is capable of transmitting a wide band offrequencies with comparatively low attenuation and which may be soshielded as to be practically immune to external disturbances, may befound in a network of circuits consisting of two conductors disposedcoaxially with respect to one another. A circuit of this type has beendisclosed in the patents to L. Espenschied and H. A. Afiel, No.1,835,031, December 8, 1931, H. A. Affel and E. I. Green, No. 1,781,092,November 11, 1930, etc.

As an alternative to the coaxial circuit the high frequency transmissionmedium might comprise a net-work of circuits, each consisting of twoparallel conductors surrounded by a shield. Circuits of this type aredisclosed in the applications of Green, Curtis and Mead, Serial No.674,762, Green and Curtis, Serial No. 674,763 and Green and Leibe,Serial No. 674,764, all filed June 7, 1933.

The invention contemplates also utilizing as the high frequencytransmission medium a dielectric wave guide comprising a cylindriformdielectric material which may or may not be surrounded by a conductor.Such dielectric guides are disclosed in the patent applications of G. C.Southworth, Serial No.661,154, filed March 16, 1933, and Serial No.701,711, filed December 9, 1933.

It is contemplated in accordance with the invention that the subscriberset and central office arrangements which have been described may beemployed with any of the above types of transmission paths. For thispurpose, it is necessary merely to substitute the transmission network.for. the radio path, replacing the connection of the apparatus to theradio antenna by a connectionsuitable to the type of medium employed.

It will be obvious from the preceding discussion that the generalprinciples herein disclosed may be embodied in many other organizationswidely different from those illustrated without departing from thespirit of the invention as defined in the following claims.

What is claimed is:

1. A common transmission medium extendin to each of a plurality of morethan two subscribers stations and to a central station, carrier currenttransmitting apparatus of a certain frequency at each subscribersstation, carrier current receiving apparatus of a certain frequency ateach subscribers station, all such frequencies being different fordifferent stations, individually means at said subscribers stations andtranslating means at said central station common to all of thesubscribers stations whereby a two-way carrier frequency signaling pathmay be established over said medium through the same translating meansat said central station between any one and any other of saidsubscribers stations, and additional common translating means includingfrequency selective apparatus whereby a plurality of such paths may beestablished and simultaneously maintained without interference betweendifferent pairs of stations.

2. A common transmission medium extending to each of a plurality of morethan two subscribers stations and to a central OfilCB, carrier currenttransmitting apparatus of a certain frequency at each subscribersstation, carrier current receiving apparatus of a certain frequency ateach subscribers station, all such frequencies being different fordifferent stations, individually means at said subscribers stations andtranslating means at said central office common to :all of thesubscribers stations whereby a two-way carrier frequency telephoneconnection may be established over said medium through the sametranslating means at said central oiiice between any one and any otherof said subscribers stations, and additional common .translating meansincluding frequency selective apparatus whereby a plurality of suchconnections may be established and simultaneously maintained withoutinterference between different pairs of stations.

3. A system whereby intelligence may be interchanged between any one andany other ofv a plurality of more than two subscribers stations througha central station, said system comprising radio transmitting equipmentof one carrier frequency and radio receiving equipment of anothercarrier frequency at each of said stations, said frequencies beingdifferent for different stations, a translating device at the centralofiice common to all of the subscribers stations including a radiotransmitter and radio receiver adjustable to the frequencies assigned toany station which is calling, and connected therewith a radiotransmitter and radio receiver adjustable to the frequencies assigned toany station which is called, whereby any subscribers station maycommunicate with any other subscribers station through the sametranslating device, and additional translating devices at the central.office whereby a plurality of two-way interchanges of intelligence maybe carriedv out simultaneously between different pairs of stations ondifferent frequency assignments.

4. A system for telephone communication between any one and any other ofa plurality of more than two subscribers stations, said systemcomprising a plurality of groups of subscribers stations and a centralstation all in readiness for connection either way through a singlecommon medium, each subscribers station comprising transmitting andreceiving apparatus adapt ed for respective fixed frequencies, thesefrequencies being different for different subscribers stations, atranslating device at the central office common to all of thesubscribers stations including a radio transmitter and radio-receiveradjustable to the frequencies assigned to any station which is calling,and connected therewith a radio transmitter and radio receiveradjustable to the frequencies assigned to any station which is called,whereby any subscribers station may communicate with any othersubscribers station through the same translation device, and additionaltranslating devices at the central office by which simultaneous two-waytelephone interconnections may take place between different pairs ofstations within the group over the common medium and through the centraloffice on difierent frequency assignments, and means whereby stations indifierent groups may be interconnected over trunk circuits connectingthe central oflices associated with the diiferent groups.

5. A system whereby telephone communication may be carried out betweenany one and any other of a plurality of more than two subscribersstations, said system comprising a common transmission medium extendingto each of said sub scribers stations and to a central office, carriercurrent transmitting apparatus of a certain frequency at eachsubscribers station, carrier current receiving apparatus of a certainfrequency at each subscribers station, all such frequencies beingdifferent for different stations, a. translating device at the centraloffice common to all of the subscribers stations including a radiotransmitter and radio receiver adjustable to the frequencies assigned toany station which is calling, and connected therewith a radiotransmitter and radio receiver adjustable to the frequencies assigned toany station which is called, whereby any subscribers station maycommunicate with any other subscribers station through the sametranslation device, and additional translating devices at the centraloffice whereby a plurality of two-way telephone connections be tweendifferent pairs of stations over said medifering from those assigned toall other subscrib ers stations and being employed both whenthe stationis a calling station and when it is a called station, and a centralstation having translating means capable of such frequency adjustmentthat any subscribers station may be put into communication with anyother subscribers station through the same translating means over thefrequencies assigned to the two stations.

7. In a system in which telephone communication between any one and anyother of a plurality of more than two subscribers stations may beeffected at carrier frequencies over a common transmission medium, andin which a plurality of such communications may be carried outsimultaneously between different pairs of stations, a plurality ofsubscribers stations and a central office, means for transmitting to thecentral office and receiving from the central office at each of saidsubscribers stations telephone signals modulated upon preassignedcarrier frequencies, the transmitting and receiving frequencies for eachsubscribers station being different from one another and from those ofany other station, and means at the central ofiice to interconnect anytwo subscribers stations, said means including arrangements fortranslating in frequency at the central office the signals received fromeach station to those received by the station with which it isconnected, said translating arrangements being capable of such frequencyadjustment that the same translating arrangement may be used tointerconnect any pair of subscribers stations.

3. In a system for the exchange of intelligence, a common transmissionmedium extending to a plurality of more than two subscribers stationsand to a central office, means at each subscribers station fortransmitting to said medium signals modulated upon a carrier frequencyand for receiving from said medium signals modulated upon a differentcarrier frequency, said frequencies differing from those assigned toother subscribers stations, and translating means at the central officecommon to all the subscribers stations whereby the signals transmittedfrom any station may be received, converted to a different frequencyposition and transmitted to any other station desired, and wherebysignals from the desired station may be received, converted andtransmited to the other station, a plurality of similar translatingmeans being provided sothat a plurality of such interchanges may becarried out simultaneously between differ nt pairs of stations.

9. In a telephone exchange system, a common transmission mediumextending to a plurality of more than two subscribers stations and to acentral ofiice, said transmission medium being capable of transmitting aplurality of carrier frequency telephone bands, means at eachsubscribers station for selecting a preassigned pair of said bands to beemployed both when the station is calling and when it is called, oneband being employed for transmitting telephone signals and the other forreceiving telephone signals, the bands assigned to each subscribersstation being different from those assigned to other subscribersstations, and translating means at the central office common to all ofthe sub scribers stations whereby the signals transmitted from anystation may be received, translated to a diiferent frequency positionand transmitted to any other station desired, and whereby signals fromthe desired station may be received, converted and transmitted to theother station, a plurality of similar translating means being providedso that a plurality of such interchanges may be carried outsimultaneously between different pairs of stations.

10. In a telephone exchange system, a transmission medium extending to aplurality of more than two subscribers stations and to a central office,means at each subscribers station for transmitting carrier frequencytelephone signals of one frequency to said medium and receiving carrierfrequency telephone signals of another frequency from said medium, saidfrequencies being different for different stations, translating means atthe central office common to all the subscribers stations whereby thecarrier frequency signals transmitted by any first subscribers stationmay be received, brought down to the voice range, translated to a newfrequency location and transmitted to any second station and likewisefrom said second station to said first station, and a plurality ofsimilar translating means at the central office whereby a plurality ofsuch interchanges may be carried out simultaneously.

11. In a system whereby a plurality of interchanges of intelligence maybe carried out between any one and any other of a plurality of more thantwo subscribers stations, a common transmission medium extending to eachof said subscribers stations and to a central oifice, transmitting meansfor one frequency and receiving means for another frequency at eachsubscribers station, said frequencies being different for differentstations, means at the central office and common to the subscribersstations whereby any subscribers station may signal an operator at thecentral office, means at the central oflice and common to thesubscribers stations whereby the number of another station may be passedto the operator, means at the central office and common to thesubscribers stations whereby the operator may complete a connection toany called station, means at the central office and common to thesubscribers stations whereby a signal may be operated at the calledstation, and means at the calling and called stations wherebyintelligence may be interchanged over the carrier frequenciesrespectively assigned thereto.

12. In a system whereby a plurality of telephone communications may becarried out between any one and any other of a plurality of more thantwo subscribers stations, a common transmission medium extending to eachof said subscribers stations and to a central ofi'ice, transmittingmeans for one frequency and receiving means for another frequency ateach subscribers station, said frequencies being different for differentstations, means at the central ofiice and 1 common to the subscribersstations whereby any subscribers station may signal an operator at thecentral office, means at the central oflice and common to thesubscribers stations whereby the number of another subscribers stationmay be passed to the operator, means at the central ofiice and common tothe subscribers stations whereby the operator may complete a connectionto the called station, means at the central ofiice and common to thesubscribers stations whereby a signal may be operated at the calledstation, and means at the calling and called stations whereby telephonecommunication may be carried out over the carrier frequenciesrespectively assigned thereto.

13. In a system whereby a plurality of telephone communications may becarried out between any one and any other of a plurality of more thantwo subscribers stations, a common transmission medium extending to eachof said subscribers stations and to a central office, transmitting meansfor one frequency and receiving means for another frequency at eachstation, said frequencies being different for different stations, meansat the central office and common to the subscribers stations whereby anysubscribers station may signal an operator at the central 0ffice bytransmitting an unmodulated carrier frequency over said medium, means atthe central office and common to the subscribers stations whereby thenumber of another subscribers station may be passed to the operator,means at the central office and common to the subscribers stationswhereby the operator may complete a connection to the called station,means at the central ofiice and common to the subscribers sta-' tionswhereby a signal may be operated at the called station by a carrierfrequency transmitted from a central ofiice, and means at the callingand called stations whereby telephone communication may be carried outover the carrier frequencies respectively assigned thereto.

14. In combination, a plurality of more than two terminal stations and acentral station with which they are permanently in readiness forconnection either way through a single common medium, carrier currenttransmitting apparatus and carrier current receiving apparatus at eachterminal station adapted to operate at respective frequencies which aredifferent from the frequencies for the other terminal stations, signalindicating means at the central station responsive respectively to thetransmitting carrier frequencies of the terminal stations, means at thecentral station to signal to the terminal stations on their respectivereceiving frequencies, and a plurality of sets of adjustable means atthe central station, each such set being adapted to convert modulatedcurrents received on their respective transmitting frequencies from eachof any two terminal stations and to transmit correspondingly modulatedcurrents on their inversely respective receiving frequencies.

LLOYD ESPENSCHIED. NEWTON MONK.

